Displaying printed sheet materials outdoors presents particular problems in protecting the displayed material from weather and vandalism. Especially where time-dated material is displayed, a holder for displaying the material must releasably support the printed material in a manner so that the material can be regularly replaced with updated material, and must protect the material. Vandalism is a particularly acute problem where printed materials are displayed in public areas. Bus schedules, for example, must be displayed at or near bus stops, typically in areas unguarded and unprotected from the weather. The sign holder which holds the bus schedule sheet must not only protect the schedule sheet from the whether, but must also discourage attempts to dislodge the schedule sheet from the holder or to mutilate the schedule sheet.
A variety of sign holders have been developed to alleviate these problems. In one design, a sign holder has walls which are grooved to slidably receive a clear face plate. Bolts secure the face plate to the sign holder so that the face plate can only be removed by first removing the bolts. The sheet material is sandwiched between the face plate and the holder. A particular problem with this type of holder is that the bolts are troublesome to handle and can be easily dropped, or misplaced. Furthermore, the type of tool required to remove the bolts is often a standard household tool, providing little security against unauthorized removal of the cover.
Another type of sign holder has been devised which utilizes a clear, flexible cover plate which is received in a base. The base is adapted to be mounted on a vertical pole or other surface. The base has side walls which extend outward from a back wall and are sized to closely receive the cover plate therebetween. The printed sheet material is positioned sandwiched between the back wall and the clear cover plate. The ends of the base have end flanges which define pockets to receive the ends of the cover plate. The base is dimensioned such that the cover plate can only be inserted or removed from the base if the cover plate is flexed into a bow shape to reduce the effective overall length of the cover plate and allow at least one of its ends to clear the corresponding end flange forming one of the pockets. When the flexing force is removed from the cover plate, the cover plate relaxes and resumes its original length and planar shape. When inserted in the base and then relaxed, the ends of the cover plate are retained in the pockets by the flanges. The cover plate is manufactured of a relatively rigid, but yet somewhat flexible clear plastic which requires a person to apply a sufficient force to bow the cover plate to remove it from the base that it is difficult to do without special tools. Because the edges of the cover plate are closely received by the sidewalls of the holder and because the ends of the cover are retained within the pockets, it is difficult or impossible to pry the cover from the base with conventional household tools.
The primary method for removing the cover plate is to depress a suction cup against the smooth front face of the cover plate and pull it outwardly. This causes the cover to flex into an outward bowed shape which frees at least one end of the cover plate from its corresponding pocket in the base. Once one end of the cover plate is free, the cover plate can be separated from the base and a sheet sign, such as a paper bus schedule, can be inserted flat against the back wall of the base. To replace the cover, one end of the cover plate is inserted in one pocket while a suction cup and a free hand are used to pull outward against the center and push inward against the freeend of the cover plate, respectively, to bow the cover sufficiently for the free end of the cover plate to clear the other flange in preparation for insertion into the flange forming the pocket. The force on the cover plate is then relieved so that it relaxes and returns to its original planar shape with both ends retained within their respective pockets. Because suction cups are not common household tools, this type of sign holder has proved to be substantially tamper proof.
A serious problem with the above type of sign holder, however, is that water tends to collect in the interface between the cover plate and the back wall and displaces air in the interface, thus causing a vacuum to be formed. Once the vacuum is formed, it becomes extremely difficult even with a suction cup to apply enough outward force on the cover plate to bow the flexible cover plate away from the back wall in order to free one end of the cover plate from its pocket. It has also been found that the problem is particularly acute when the sheet material positioned between the cover plate and back wall is made of a paper, which tends to absorb and wick water into the interface between the cover and base.